Refactor SVG sanitizer and prevent `xlink:href` to refer to external resources

lib/gitlab/sanitizers/svg.rb

@@ -10,13 +10,19 @@ module Gitlab
         DATA_ATTR_PATTERN = /\Adata-(?!xml)[a-z_][\w.\u00E0-\u00F6\u00F8-\u017F\u01DD-\u02AF-]*\z/u
         def scrub(node)
-          unless Whitelist::ALLOWED_ELEMENTS.include?(node.name)
-            node.unlink
-          else
+          if Whitelist::ALLOWED_ELEMENTS.include?(node.name)
             valid_attributes = Whitelist::ALLOWED_ATTRIBUTES[node.name]
+            return unless valid_attributes
             node.attribute_nodes.each do |attr|
-              unless valid_attributes && valid_attributes.include?(attribute_name_with_namespace(attr))
+              attr_name = attribute_name_with_namespace(attr)
+
+              if valid_attributes.include?(attr_name)
+                # xlink:href is on the whitelist but we should deny any reference other than internal ids
+                if attr_name == 'xlink:href' && unsafe_href?(attr)
+                  attr.unlink
+                end
+              else
                 if Whitelist::ALLOWED_DATA_ATTRIBUTES_IN_ELEMENTS.include?(node.name) && data_attribute?(attr)
                   # Arbitrary data attributes are allowed. Verify that the attribute
                   # is a valid data attribute.
@@ -26,6 +32,8 @@ module Gitlab
                 end
               end
             end
+          else
+            node.unlink
           end
         end
@@ -37,7 +45,9 @@ module Gitlab
           end
         end
-        private
+        def unsafe_href?(attr)
+          !attr.value.start_with?('#')
+        end
         def data_attribute?(attr)
           attr.name.start_with?('data-')

spec/lib/gitlab/sanitizers/svg_spec.rb

@@ -2,12 +2,12 @@ require 'spec_helper'
 describe Gitlab::Sanitizers::SVG do
   let(:scrubber) { Gitlab::Sanitizers::SVG::Scrubber.new }
-  let(:namespace) { double(Nokogiri::XML::Namespace, prefix: 'xlink') }
-  let(:namespaced_attr) { double(Nokogiri::XML::Attr, name: 'href', namespace: namespace) }
+  let(:namespace) { double(Nokogiri::XML::Namespace, prefix: 'xlink', href: 'http://www.w3.org/1999/xlink') }
+  let(:namespaced_attr) { double(Nokogiri::XML::Attr, name: 'href', namespace: namespace, value: '#awesome_id') }
   context 'scrubber' do
     describe '#scrub' do
-      let(:invalid_element) { double(Nokogiri::XML::Node, name: 'invalid') }
+      let(:invalid_element) { double(Nokogiri::XML::Node, name: 'invalid', value: 'invalid') }
       let(:invalid_attribute) { double(Nokogiri::XML::Attr, name: 'invalid', namespace: nil) }
       let(:valid_element) { double(Nokogiri::XML::Node, name: 'use') }
@@ -44,5 +44,17 @@ describe Gitlab::Sanitizers::SVG do
         expect(scrubber.attribute_name_with_namespace(namespaced_attr)).to eq('xlink:href')
       end
     end
+
+    describe '#unsafe_href?' do
+      let(:unsafe_attr) { double(Nokogiri::XML::Attr, name: 'href', namespace: namespace, value: 'http://evilsite.example.com/random.svg') }
+
+      it 'returns true if href attribute is an external url' do
+        expect(scrubber.unsafe_href?(unsafe_attr)).to be_truthy
+      end
+
+      it 'returns false if href atttribute is an internal reference' do
+        expect(scrubber.unsafe_href?(namespaced_attr)).to be_falsey
+      end
+    end
   end
 end